The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950° C. is more than one hundred years old. This process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.
U.S. Pat. No. 3,400,061 (Lewis/Hildebrandt) and U.S. Pat. No. 4,602,990 (Boxall/Gamson/Green/Traugott) disclose aluminium electrowinning cells with sloped drained cathodes facing anodes sloping across the cell. In these cells, the molten aluminium flows down the sloping cathodes into a median longitudinal groove along the centre of the cell, or into lateral longitudinal grooves along the cell sides, for collecting the molten aluminium and delivering it to a sump.
In U.S. Pat. No. 5,362,366 (de Nora/Sekhar), a double-polar anode-cathode arrangement was disclosed wherein cathode bodies were suspended from the anodes permitting removal and reimmersion of the assembly during operation, such assembly also operating with a drained cathode.
U.S. Pat. No. 5,368,702 (de Nora) proposed a novel multimonopolar cell having upwardly extending cathodes facing and surrounded by or in-between anodes having a relatively large inwardly-facing active anode surface area. In some embodiments, electrolyte circulation was achieved using a tubular anode with openings.
U.S. Pat. No. 5,651,874 (de Nora/Sekhar) proposed coating components with a slurry-applied coating of refractory boride, which proved excellent for cathode applications. This publication discloses slurry-applied applications and novel drained cathode configurations, including designs where a solid cathode body with an inclined upper drained cathode surface is placed on or secured to the cell bottom.
U.S. Pat. No. 5,472,578 (de Nora) discloses an aluminium production cell comprising a grid on the cell bottom for restraining motion of the aluminium pool on the cell bottom. In some embodiments, the top end of the grid forms an aluminium-wettable drained cathode surface under an active anode surface.
WO00/40782 (de Nora) discloses aluminium production anodes with a series of coplanar parallel elongated anode members which are spaced-apart by flow-through openings and which form an electrochemically active surface. In one embodiment two downwardly converging spaced apart adjacent anodes can be arranged between a pair of substantially vertical cathodes. The adjacent anodes are spaced apart by an electrolyte down-flow gap in which alumina-rich electrolyte flows downwards until it circulates via the adjacent anodes' flow-through openings into the inter-electrode gaps.
WO01/31088 (de Nora) discloses aluminium electrowinning cells with solid anodes having a V-shaped active surface facing sloping cathodes. The anodes and cathodes are associated with vertical passages for the circulation of alumina-rich electrolyte to a bottom part of the inter-electrode gaps spacing the anodes and cathodes.
While the foregoing references indicate continued efforts to improve cell operations, none suggests the invention and there have been no entirely acceptable proposals for improving the cell efficiency, and at the same time facilitating the implementation of a drained cathode configuration with improved electrolyte circulation and large storage capacity of product aluminium.